Method of producing large metal casting cores



March 24, 1964 N. URONES ETA| 3,125,787

METHOD OF PRODUCING LARGE METAL CASTING CORES Filed Feb. 14, 1961 mmvroks Theodore Operha United States Patent of Delaware Filed Feb. 14, 1961, Ser. No. 89,230 7 Claims. (Cl. 22--194) This invention relates to new metal casting cores useful in the casting of high melting point metals such as those contained in group IVB of the periodic table. Specifically, the invention relates to large metal casting cores which may be employed in the casting of such metals in ceramic shell metal casting molds.

In the propulsion of high speed aircraft which uses as part of the propulsion system turbines and similar type devices, problems have arisen in connection with the materials used to produce parts for such systems that are capable of withstanding the extremes of high temperature and friction which quite commonly occur. To a lesser extent, the problem also occurs in the newer high pressure, high speed gas and steam turbines.

In order to combat these problems of high heat and frictional disturbances, designers and manufacturers have utilized such high melting point metals as titanium, zirconium, and hafnium. Even though such high melting point materials have been used, the temperatures now encountered in some of the newer aircraft and turbine equipment tend to cause these materials to fail. To overcome the problem, further modifications in equipment design have been made, particularly in turbine blade construction. In such devices, it is now common practice to place a series of vents or passages running throughout the interior surfaces of such devices in an attempt to effectuate air cooling which would keep these metal structures from overheating.

When designing such air cooled type structures, it is desirable to obtain the maximum surfaces through which air can conduct heat away, yet, at the same time, not structurally Weaken the component.

In order to produce air cooled structures of the type described above, particularly turbine components, where high melting point metals are used as materials of construction, it. has frequently been the practice of the art to resort to manufacturing these products using a lost wax casting technique. In a preferred manufacturing operation, such parts are made utilizing the special type lost wax casting process generally known as a ceramic shell metal casting process.

It was soon found that the production of openings in turbine components was a difiicult problem to solve using a ceramic shell molding process because of the diificulties experienced in making cores for the molds. A primary area of difficulty consists of the need for finding a suitable core material which could withstand the rigors of the casting process, and a particularly troublesome problem is presented where large cores are used in producing high melting point castings. They are frequently difficult to produce, so that close tolerances are maintained and quite often their size causes them to break during the various mold processing steps.

In our co-pending application for Metal Casting Cores, filed on or about this same date, we have shown that suitable casting cores for use in producing high melting point metal castings by the ceramic shell metal casting process may be produced by coating onto a core base a uniform film, preferably a thin uniform film of a high melting point, difiicultly oxidizable metal or alloy.

The interior portion of the core may be composed of any type solid substance which is capable of accepting a R 3,125,787. Patented Mar. 24, 1964 film of the high melting point, difficultly oxidizable metal. Thus, for instance, the core interior may be composed of either metals or non-metals, although metals are preferred. Such non-metallic substances as low melting point ceram- ICS, waxes and the like may be used as the core base. In the case of metals, any metal which has a melting point lower than the melting point of the high melting point diflicultly oxidizable metal outer coating may be used. Thus, such metals as copper, iron, steel, brass, admiralty metal, tin, silver, bismuth and the like may be used.

In a preferred embodiment in accordance with said copending application, it is desired to use as the inner portion of the core molybdenum which possesses special properties that make it the most satisfactory under most conditions.

The high melting point difficultly oxidizable metal employed as the outer surface coating of the core is preferably selected from those metals of this particular class which have a boiling point and/or melting point higher than the temperature of the metal poured into the mold. Thus, with such materials as titanium, which are frequently poured at temperatures ranging from 2800 F. to 3100 F., it is desirable to use as the outer coating such metals as chromium, platinum, tungsten, columbium, and tantalum.

While the core system described in said application is imminently suitable for casting cores useful in the production of high melting point metal parts such as those of group IV-B metals, it Was found that when large size cores were made the process had some limitations from the standpoint of the time involved in producing the cores as Well as the fact that larger cores tended to lose some of their dimensional tolerance during the core making process.

It is, therefore, an object of this invention to produce a new type core for use in casting high melting point metals and alloys, such as the metals and alloys of group IVB, particularly using as the casting process a ceramic shell mold process.

Another object is to provide new type casting cores of the type described above which are capable of withstanding temperatures of such casting processes without deforming or being oxidized to a gaseous state and which are easily removed from the finished piece.

It is an additional object of this invention to provide a method capable of achieving the above objects and which is adaptable for use in connection with cores of relatively large size.

These and other objects of this invention will appear hereinafter and for purposes of illustration, but not of limitation, specific embodiments of this invention are shown in the accompanying drawings in which FIGURE 1 is a perspective view of a core of the invention which is shown encased in an expendable pattern.

FIGURE 2 is a cross sectional view across the line 22 of FIGURE 1, and illustrates the cores of this invention as containing an inner coating between an outer shell of high melting point difiicultly oxidizable metal and the interior of a core.

In accordance with the present invention, we have found that large size cores having the noted characteristics may be satisfactorily produced by using the expediency of coating an expendable pattern with the high melting point difficultly oxidizable metal, removing the expandable pattern, which leaves a shell of the coated metal, and then filling the shell with certain solid substances to provide rigidity to the finished core. In a preferred embodiment, the expandable pattern is first coated with either graphite or molybdenum about which the high melting point difficultly oxidizable metal film is placed. The expendable patterns'used to form the cores of this invention may be selected from such well known expandable pattern substances as wax and thermo plastics of which polystyrene, polyvinyl chloride, polyvinylidene chloride and the like are exemplary. Also useful as pattern materials for forming the cores are low melting point metals and alloys which may be removed from the outer coated shell by the application of low temperatures. It is preferred to use wax as the core pattern forming material. When wax is so used, it may be removed from the metallic film coated about it by using either heat or solvents such as low molecular weight chlorinated parafiinic hydrocarbons. The high melting point, difiicultly oxidizable metals which are coated about the expendable pattern are the same as those used in our core forming process and cores previously described. Here again, the preferred metal is chromium which may be conveniently coated around the pattern material by using such expediencies as vacuum electro-deposition, time spraying, modified chrome plating techniques and the like.

In forming the coating of the high melting point difficultly oxidizable metal about the pattern, it is important that the coating be of sufficient thickness such that when the expandable pattern is removed, the core will be capable of forming a self-supporting structure. Therefore, the coating should have a minimum thickness of about .002 thousandth of an inch with thicknesses ranging from .005 thousandth of an inch and above being suitable for most cores.

After the expandable pattern has been removed from the self-supporting film of high melting point, dimcultly oxidizable metal, it is then filled with a solid substance which has a melting point in excess of 1000 F. The solid substance may be poured into the self-supporting film as a liquid, although in most cases, it will be distributed throughout the interior of such shell by using the filler material in the form of a finely divided solid.

The materials used to fill the thin shell or" metal which forms the exterior of the cores may be selected from such substances as metals, ceramics and non-metals such as graphite, silicon and the like with the proviso that the particular substance used should have a normally solid state at temperatures in excess of 1000 F.

In a preferred embodiment, the filling material which is contained in the interior of the thin shell of the core is composed of either molybdenum or graphite.

As previously mentioned, in the preferred embodiment of this invention, the pattern is first coated with a film of either molybdenum or graphite before the coating of high melting point difiicultly oxidizable metal is applied. This particular feature enables a more rigid and structurally strong core to be formed. It also allows advantage to be taken of the superior core qualities of graphite and molybdenum when the substances have been rendered non-gas forming by an outer protective metal shell. When such primary or base coatings of graphite or molybdenum are utilized, it is desirable that they have a thickness of suflicient dimension such that they may be removed from between the outer coating of the high melting point, difiicultly oxidizable metal and the interior filling of the core without difiiculty. This is particularly true when high melting point metals are used as the filler for the cores.

Thus, when the coating placed about the expendable pattern is a two-layer system of the type described above, it is possible to fill the interior of the core with a high melting point metal which, in a preferred embodiment, is a metal which corresponds to the metal used to form the outer surface coating of the core.

It will thus be apparent to those skilled in the art that the middle or intermediate layer of the graphite or molybdenum must be sufiiciently thick so that it forms a parting area between the outer shell and the inner filling of the core when high melting point fillers are used, e.g., a graphite coating .0002 of an inch thick on the inner surface of an .005 inch thick chromium outer coating is suitable.

Referring to the drawings, in FIGURE 1, the numeral 10 represents a wax pattern which is generally in the shape of a blade which might be used in the production of a turbine. This pattern contains an enlarged sprue piece 118 which is formed onto the pattern and which, when coated with a ceramic in a shell molding process, forms an opening into which the metal will be poured. Contained within the pattern is a core 12 which is comprised of an outer shell 13 composed of a high melting point, difiicultly oxidizable metal and a self-supporting filling material 14. It will be noted that the tip end 20 of the core extends beyond the end 22 of the pattern which feature allows the core to become rigidly aifixed to the ceramic coatings which are subsequently stuccoed around the pattern. The particular core shown contains a plurality of openings 16 which tend to uniformly distribute the heat throughout the various processes conducted in manufacturing the casting molds as well as providing a plurality of supports for the interior of the finished piece.

The preferred cores of the invention are illustrated in FIGURE 2, wherein it is shown that the core 12 contains an outer shell 13 which is formed over an inner coating 26 which, as previously indicated, is preferably composed of graphite or molybdenum. The interior of the core has been filled with a finely divided particulate substance such as ceramic, finely divided metal and the like which is illustrated by the numeral 14. It will, of course, be understood that while the filling or the interior portion of the core has been shown to be composed of finely divided particles, it may be made of a completely solid substance.

The filling material used in producing these cores lends suificient body to the core so that it is dimensionally stable and rigid and will not be deformed during the physical handling operations which must be performed on the core. Also, the interior filling material, when selected of the proper substance, has the additional advantage of providing the core with the ability of being used in the melting out and subsequent high temperature operations without being deformed.

While the cores of the invention are most suitably used in such processes as ceramic shell metal casting processes, it will be understood that they are capable of use in any type of casting process wherein high melting point metal objects are produced. However, in order to provide a specific example of a casting process suitable for use with the cores of this invention, reference is made to an application Serial No. 708,628 (now Patent No. 2,961,751), filed January 13, 1958.

It will be understood that various modifications may be made in the above disclosed process which provide the characteristics of this invention without departing from the spirit of the invention, particularly as defined in the following claims.

We claim:

1. A method for the production of cores which comprises the steps of forming an expendable pattern which has the general shape of the core sought to be produced, coating said pattern with molybdenum, providing an additional coating of a high melting point, difiicultly oxidizable metal, which has a boiling point above the casting temperature of the metal to be cast with said cores, said difficultly oxidizable metal being selected from the group consisting of chromium, platinum, tungsten, columbium and tantalum, said coatings being of suificient thickness to be self-supporting, removing the expandable pattern from the coatings and filling the interior thereof with a solid substance which is capable of remaining in a solid state at temperatures in excess of 1000 F.

2. The process of claim 1 wherein said solid substance comprises molybdenum.

3. The process of claim 1 wherein said solid substance comprises graphite.

4. The process of claim 1, wherein said solid substance is comprised of a metal corresponding to the metal used to form the self-supporting high melting point, difiicultly oxidizable metal coating.

5. The process of producing a core useful in producing metal pieces from group IV-B metals and alloys thereof by ceramic shell process which comprises the steps of forming about an expendable pattern having the general shape of the cores sought to be produced a coating of molybdenum, forming an additional coating of a high melting point, difiicultly oxidizable metal, which has a boiling point above the casting temperature of the metal to be cast with said cores, said difficultly oxidizable metal being selected from the group consisting of chromium, platinum, tungsten, columbiurn and tantalum, said coatings being of sufiicient thickness to be self-supporting, removing the expendable pattern from the coatings and filling the interior thereof with a solid substance which is capable of remaining in a solid state at temperatures in excess of 1000 F.

6. A method for the production of cores which comprises the steps of forming an expendable pattern which has the general shape of the core sought to be produced, coating said pattern with graphite, providing an additional coating of a high melting point, diflicultly oxidizable metal, which has a boiling point above the casting temperature of the metal to be cast with said cores, said difficultly oxidizable metal being selected from the group consisting of chromium, platinum, tungsten, columbium and tantalum, said coatings being of suflicient thickness to be self-supporting, removing the expendable pattern from the coatings and filling the interior thereof with a solid substance which is capable of remaining in a solid state at temperatures in excess of 0" F.

7. The process of producing a core useful in producing metal pieces from group IV-B metals and alloys thereof by ceramic shell process which comprises the steps of forming about an expendable pattern having the general shape of the cores sought to be produced a coating of graphite, forming an additional coating of a high melting point, difficultly oxidizable metal, which has a boiling point above the casting temperature of the metal to be cast with said cores, said difficultly oxidizable metal being selected from the group consisting of chromium, platinum, tungsten, columbiurn and tantalum, said coatings being of suflicient thickness to be self-supporting, removing the expendable pattern from the coatings and filling the interior thereof with a solid substance which is capable of remaining in a solid state at temperatures in excess of 1000 F.

References Cited in the file of this patent UNITED STATES PATENTS 1,912,889 Couse June 6, 1933 11,935,916 Ragsdale Nov. 211, 1933 2,629,907 Hugger Mar. 3, 1953 2,806,271 ()perhall Sept. '17, 1957 2,880,486 Wallace Apr. 7, 1959 

1. A METHOD FOR THE PRODUCTIN OF CORES WHICH COMPRISES THE STEPS OF FORMING AN EXPENDABLE PATTERN WHICH HAS THE GENERAL SHAPE OF THE CORE SOUGHT TO BE PRODUCED, COATING SAID PATTERN WITH MOLYBDENUM, PROVIDING AN ADDITIONAL COATING OF A HIGH MELTING POINT DIFFICULTY OXIDIZABLE METAL, WHICH HAS A BOILING POINT ABOVE THE CASTING TEMPERATURE OF THE METAL TO BE CAST WITH SAID CORES, SAID DIFFICULTLY OXIDIZABLE METAL BEING SELECTED FROM THE GROUP CONSISTING OF CHROMIUM, PLATINUM, TUNGSTEN, COLUMBIUM AND TANTALUM, SAID COATINGS BEING OF SUFFIECIENT THICKNESS TO BE SELF-SUPPORTING, REMOVING THE EXPANDABLE PATTERN FROM THE COATINGS AND FILLING THE INTERIOR THEREOF WITH A SOLID SUBSTANCE WHICH IS CAPABLE OF REMAINING IN A SOLID STATE AT TEMPERATURES IN EXCESS OF 1000*F. 